An organic semiconductor is expected to be applied to a photoelectric conversion element such as an organic thin film solar cell, a light emitting element, and a photosensor. Especially, a high molecular compound as an organic semiconductor material enables application of a low-cost coating method in fabrication of an active layer. In view of an energy demand and an emission reduction of CO2, a solar cell is expected as one of clean energies with low environmental burdens and its demand is rapidly increasing. A silicon-based solar cell is mainstream at present, but an efficiency thereof is about 15%, and it is difficult to curtail a cost. A CdTe solar cell is also known as a solar cell which can be fabricated at a low cost. However, since Cd being a toxic element is used, there is a possibility that an environmental problem occurs. Under the circumstances, development of an organic thin film solar cell is increasingly expected as a next-generation solar cell which is low in cost, high in energy conversion efficiency, and nontoxic.
In order to put an organic thin film solar cell to practical use, improvement of a power generation efficiency is intensely demanded. The organic thin film solar cells using organic semiconductors made of a variety of high modular compounds are studied. For example, as a polymer exhibiting a conversion efficiency as high as 7% or more, which is at top level in the world, when applied to an organic thin film solar cell, there is known poly(4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxycarbonyl)-5-fluoro-thieno[3,4-b]thiophene-2,6-diyl) (abbreviated form: PTB-7). Further, there is also known a polymer (abbreviated form: PBDTTT-CF) made by substituting a carbonyl group (n-heptylcarbonyl group) which is strong in electron attracting for an ester substituent (2-ethylhexyloxycarbonyl group) at 4-position of a thieno[3,4-b]thiophene ring of PTB-7, in order to improve an open circuit voltage (Voc) of an organic thin film solar cell.
In a synthetic process of the aforementioned polymers (PTB-7 and PBDTTT-CF), synthetic pathways of monomers are multi-stepped, and the synthetic process are quite complicated. In order to decrease synthetic stages, there is developed poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-4-(2-heptylcarbonyl)-thieno[3,4-b]thiophene-2,6-diyl} (abbreviated form: PDBTTT-C) in which hydrogen is substituted for fluorine of 5-position of thieno[3,4-b]thiophene of the PBDTTT-CF. This polymer has a problem that a conversion efficiency is decreased. With regard to the PDBTTT-C, it is known that the conversion efficiency is improved by replacing a 2-ethylhexyloxy group of substituents of 4-position and 8-position of a benzo[1,2-b:4,5-b′]dithiophene ring with a 5-(2-ethylhexyl)thienyl group. A conversion efficiency of a solar cell is largely affected also by a side chain of a polymer.
With regard to improvement of a framework structure of a polymer, there is reported a 2,7-dithia-5-azacyclopenta[a]pentalene-4,6-dione based polymer, in which an imide ring is condensed at 4-position and 5-position of the above-described thieno[3,4-b]thiophene. An open circuit voltage (Voc) of an organic thin film solar cell using this polymer is about 0.72 V, and a conversion efficiency stays to be about 5% or less. In order to improve a performance of an organic thin film solar cell using an organic semiconductor, it is necessary to reform a skeletal structure or a side chain of a polymer.